A treatment target may move during image guided treatment such as radiation treatment. To account for such movement, traditional gantry-based radiation delivery systems treat the Internal Target Volume (ITV), i.e., the entire range of motion of the target during respiration or other movement, with a margin of expansion to compensate for setup inaccuracy, gross patient movement during treatment, and change of breathing pattern or other motion pattern between planning and treatment. This approach, however, comes with the drawback of large volumes of normal tissue being exposed to the prescription dose, especially in cases where the tumor undergoes a large excursion during breathing.